The evolution of telecommunication during the last few decades has involved changes both in the access network and in the core/services network. Earlier fixed landline telecommunication systems as well as wireless communication systems provided circuit switched (CS) telephone calls and, to a limited extent, data communication between subscribers. Later wireless communication systems have evolved into very advanced digital data communication systems where telephone, i.e. voice, calls as well as data connections and services associated with data connections are based on internet protocol (IP) standards. Much work regarding standardization of the evolution has taken place within the third generation partnership project (3GPP) and Long Term Evolution (LTE) and IP Multimedia Subsystem (IMS) are respectively major examples of the 3GPP work.
LTE, with the enhanced packet core (EPC), is the natural upgrade path for carriers with both Global System for Mobile Communications/Universal Mobile Telecommunications System (GSM/UMTS) networks as well as Code division multiple access 2000 (CDMA2000) networks. IMS core network is the evolution of Mobile Softswitch Solution core network (MSS) where the telephony services are provided through AS (Application Server). An IMS network is able to provide telephony services regardless of access technology, although when LTE access technology is used, a high quality of service can be guaranteed especially for real-time services.
Voice over LTE (VoLTE) service has been introduced to allow voice service through fourth generation (4G) networks (i.e. LTE and EPC) by means of IMS, since the fourth generation mobile is a packet-only network without the CS core. A VoLTE call can be seen as an IMS voice over IP (VoIP) call using the LTE access network.
In general IMS multimedia services are based on two main protocols: session initiation protocol (SIP) for control signaling and Real-Time Protocol (RTP) for media streams. So from a lawful Interception (LI) point of view the interception of a VoLTE call or other IMS services, into IMS domain, means interception of SIP signaling and interception of related RTP streams.
VoLTE technology is able to carry a speech call on an IMS network giving to the end user an improved experience in comparison with a traditional CS call. This is essentially due to the fact that SIP signaling is more flexible than the previous SS7 signaling used in CS networks and also due to the fact that the RTP protocol allows use of coders/decoders (codecs) that are more efficient and provide a better quality of service than circuit switched voice services.
From a lawful interception point of view, however, there are some drawbacks related to VoLTE calls. Specifically, law enforcement agencies (LEA) should be able to rebuild an intercepted call from handover interface (HI) information provided by a Mediation Device. HI port 3 (HI3) flow carries RTP packets whilst HI port 2 (HI2) includes SIP signaling and some other information to be used to decode RTP packets and rebuild the voice during the call. However, lawful interception standards (e.g. any European Telecommunications Standards Institute (ETSI), 3GPP or American National Standards Institute (ANSI)) do not foresee the presence of all the useful information needed to decode the received VoIP payload inside HI3 headers. This obliges the LEA to implement heavy post processing business logic to work around this lack of information on the HI3 interface.
This drawback is further stressed considering that the different providers of wireless communication devices can use different RTP codecs. Also, the codec may also be changed by various network elements during a VoLTE call. Since codec information is spread over the HI2 and HI3 interfaces, the LEA needs to buffer HI2 and HI3 packets in order to be able to post-process the packets when the intercepted call has been terminated. The result is that from a lawful interception point of view there is a degraded service in case of VoLTE call, in contrast to the service that is provided when intercepting traditional CS calls. In this context, a prior art publication is the 3GPP technical specification TS 33.108 V12.12.0.